docs: Mild fix to a sensors_overview code example
docs: Mild fix to a sensors_overview code example

In the previous version, if a device had both a gravity sensor and
an accelerometer, but did not have the Google Inc. version 3 gravity
sensor, we would end up using no sensor at all.  We change this to
have the code use an accelerometer in this case, which more closely
matches the surrounding description text.

Change-Id: I095b94d0d8207d3106db022dc337dd537ee0fa82

xorware/android_frameworks_base,xorware/android_frameworks_base,xorware/android_frameworks_base,xorware/android_frameworks_base,xorware/android_frameworks_base,xorware/android_frameworks_base