health: drop backup instance for non-recovery.

For the core variant (system), clients like storaged
etc. should no longer access the "backup" instance, because
healthd is removed from the system image.

For the vendor variant, this would only break if a
vendor image were launched with Android O / O-MR1 (so
health@2.0 is not required), and were compiled against an
Android T (and above) source tree (which won't check
the "backup" instance). This shouldn't be a valid configuration.

For the recovery variant, the "backup" instance has a different
meaning. It points to

  android.hardware.health@2.0-impl-default.recovery

... which was assumed by OEMs to be always installed when a
vendor-specific libhealthd is not necessary. Hence, its behavior
is kept. See

  hardware/interfaces/health/2.0/README.md

android.hardware.health@2.0-impl-default.recovery, and subsequently the
special handling of recovery mode below, can be removed once health@2.1
is the minimum required version (i.e. compatibility matrix level 3 is
removed). Health 2.1 requires OEMs to install the implementation
to the recovery partition when it is necessary (i.e. on non-A/B devices,
where IsBatteryOk() is needed in recovery).

Test: pass
Bug: 203245871

Change-Id: Ife14f5bdaba20e4cf5bd0b954aea4d04a62009c4