Renaming gpsMode to locationMode.

The constant represented location mode settings, so it makes sense to
rename it to locationMode. I left the BatterySaverPolicy key as gps_mode
since that's been there for several releases.

Bug: 79580230
Bug: 119261320
Test: atest com.android.server.power.batterysaver.BatterySaverPolicyTest
Test: atest com.android.server.power.batterysaver.BatterySavingStatsTest
Test: atest CtsBatterySavingTestCases
Change-Id: Ib550a961aaa10d683a9b4302b3dc3c0f04841fc0