Compare only 20 characters when find label on watch

On small screen devices, such as watch,
the last characters on permission lable will be omitted with ...
for example SMS, MMS.
If compare the full strings to call findByText,
the node will not be found.

When applying this patch, the case
android.appsecurity.cts.PermissionsHostTest#testNoResidualPermissionsOnUninstall
of module CtsAppSecurityHostTestCases will be passed.

Bug: 140327294

Change-Id: I798843be3b4d621c9531c42695db15284e544759
Signed-off-by: huxiandong <huxiandong@xiaomi.com>