[metrics] Change -Xwrite-metrics-to-statsd to take explicit argument

Previously, we enabled statsd reporting by checking for the presence
of the -Xwrite-metrics-to-statsd argument. This makes it so we cannot
turn it off if we change to on by default. This CL changes the
argument to be -Xwrite-metrics-to-statsd=_, which allows us to force
the setting to either on or off.

Test instructions:

    adb shell setprop dalvik.vm.extra-opts \
        -Xwrite-metrics-to-statsd=true
    adb shell stop && adb shell start
    statsd_testdrive 332

Test: manual, see above
Bug: 170149255
Change-Id: Ie43618cf0b454b0986119fc6e501c035f971af15
3 files changed