Do not create EMPTY batterystats history file.

Do not create EMPTY batterystats history file.

Previously when batterystats history buffer exceeds MAX_HISTORY_BUFFER
size, we create a new history file which is empty until batterystats
history buffer is written to the file. But the buffer is written to the
file every 30 minutes by default. When the file is empty, if the
system server process crashed or abrupt powered off, the file will remain
empty.

During the device bootup, batterystats reads history buffer from
the empty file and failed. The important mHistoryBaseTime variable is zero,
which causes all timestamps in subsequent history events are wrong, this
causes volta go/powerbug displays wrong, also Battery Usage in Settings
are wrong.

The fix is to not create new empty history file on disk until we actually
write history buffer to the file.

This problem can be manually reproduced by creating new empty file under
battery-history directory, then power off the device by long press power
button.

Bug: 133525277
Test: frameworks/base/core/tests/coretests/src/com/android/internal/os/BatteryStatsHistoryTest.java

Change-Id: I0c22881df6897e8832b472cc5e82fbf2727eb252